UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

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Now showing 1 - 6 of 6
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    Understanding organic and conventional management programs and rhizosphere microbiome for sports turf in Maryland
    (2023) Peddigari, Shravya; Carroll, Mark; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In response to public concerns about exposure to pesticides, some state and local municipalities have placed restrictions on the use of pesticides on athletic fields. When such restrictions are implemented athletic field management often transitions to the use of natural or organic turf care with little understanding of how the transition away from conventional management practices may affect surface conditions and soil microbial properties.This thesis examined the use of organic and conventional management programs on the turf quality, surface hardness, and shear strength of engineered soil cap, hybrid bermudagrass (Cynodon dactylon x Cynodon transvaalensis) athletic fields, as well as the impact of the two programs on the rhizosphere microbiome. Turf quality was assessed by visual means and by obtaining normalized difference vegetative index (NDVI) readings of the turf canopy. Surface hardness was determined using a Clegg impact surface tester. The rotational shear strength of the surface was measured using a shear vane. The study was conducted for 3 years at two different locations; research plots at the University of Maryland Research Facility and on athletic fields at Laytonia Recreational Park, in Gaithersburg, MD. Surface property data was collected monthly. Turf visual quality and NDVI data revealed use of the organic management program led to higher visual quality during spring, which was primarily the result of the spring retention of fall overseeded intermediate ryegrass (Lolium x hybridum Hausskn) and early season use of natural based fertilizers. In the summer months, crabgrass (Digitaria ischaemum Schreb.) encroachment was limited to the organically managed turfgrass. At both locations, clover (Trifolium repens) encroachment developed by the third year of the study, but the presence of this weed had limited impact on turfgrass quality. There were few significant differences in surface hardness and shear strength between the two management practices over the entire study period. The rhizosphere microbiome data, which was collected 12, 20, and 24 months after the initiation of two programs, did not show any significant difference between the organic and conventional management athletic fields in microbial abundance and/or diversity. The results of this study emphasize that the adoption of organic management programs on bermudagrass athletic fields should, in most instances, center on the establishment of acceptable weed tolerance levels for these fields. The use of organic management programs in the transition zone offers a viable alternative to the conventional chemical management of athletic fields, however over time, growing weed seed banks may necessitate the need for the occasional use of conventional herbicide materials.
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    Prevalence of Salmonella on Laying Hen Farms and Control of Colonization in Poultry Through Egg Yolk Antibodies
    (2014) Almario, Jose Alejandro Navarro; Biswas, Debabrata; Animal Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In the United States, rates of foodborne illness caused by Salmonella have not changed significantly. One study in this thesis estimated Salmonella prevalence and antimicrobial resistance of various samples from conventional (n=181) and organic (n=252) farms. Rates of Salmonella contamination were significantly lower on conventional than organic farms. Antimicrobial resistance was significantly higher on isolates from conventional versus organic farms. These findings suggest that poultry production practices may have significant effects on prevalence and antibiotic resistance patterns of Salmonella. The other study assessed the efficacy of a Salmonella control strategy using anti-Salmonella antibodies, two chicken cell lines, an HD-11 macrophage and a DF-1 fibroblast line, and Salmonella serovars Typhimurium and Enteritidis. In DF-1 cells, treatment showed decrease adherence of the pathogen. However, in HD-11 cells, treatment showed an increase in pathogen adherence, indicating a more detailed understanding of chicken response to treatment with the antibodies is needed before full-scale implementation.
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    Modeling and simulation of organic molecular clusters and overlayers on solid surfaces
    (2011) Liu, Qiang; Weeks, John D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Driven by the rapid development of experimental methods and technology, nano scale physics and chemistry has become more and more important and practicable to study. Monolayers of organic molecules have been studied a lot recently because of many potential applications, such as organic photovoltaic devices (OPV) or organic Liquid Electric Diodes (OLED). It is important to understand and interpret these new experimental advances. At molecular scales, Monte Carlo (MC) simulations and molecular dynamics (MD) are two important methods in computational chemistry and materials science. This dissertation will use these simulation methods along with statistical mechanical theory to study the behavior of single monolayers of organic molecules on solid surfaces. First we give a brief introduction to two dimensional molecular systems. Different from bulk system or single molecules, 2D systems have many unique properties, and attract much experimental and theoretical research attention. Some common methods in experimental and theoretical studies are reviewed. After introducing the properties and experimental results of ACA/Ag(111), we build a lattice gas model and run Monte Carlo simulations to help interpret the experiments. The Pair approximation, a generalization of mean-field theory, is used to calculate the global phase diagrams and put our model into the more general class of spin-1 Ising models. The pair approximation can be used for modeling various monolayer organic molecular systems which correspond to different regions of the parameter space. Then we studied the C60/ZnPc/Ag(111) system, using molecular dynamic simulations. The C60 molecules form unusual chain structures instead of the close packed islands seen on metal surfaces, and we try to provide a theoretical explanation. Finally we use a density functional theory software to calculate the electronic structures of the C60/ZnPc/Ag(111) systems. This calculation predicts a 0.4e charge transfer from substrate to C60 molecule, which we believe is important for the C60 interactions on these surfaces. In general this thesis studies the behavior of organic monolayers and bilayers on metal substrates. This basic work could help us to understand general 2-D system dynamics and electronic properties, and may help us to find new interesting systems with special properties and applications.
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    INTERFACE EFFECTS ON NANOELECTRONICS
    (2009) Conrad, Brad Richard; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nanoelectronics consist of devices with active electronic components on the nanometer length scale. At such dimensions most, if not all, atoms or molecules composing the active device region must be on or near a surface. Also, materials effectively confined to two dimensions, or when subject to abrupt boundary conditions, generally do not behave the same as materials inside three dimensional, continuous structures. This thesis is a quantitative determination of how surfaces and interfaces in organic nanoelectronic devices affect properties such as charge transport, electronic structure, and material fluctuations. Si/SiO2 is a model gate/gate dielectric for organic thin film transistors, therefore proper characterization and measurement of the effects of the SiO2/organic interface on device structures is extremely important. I fabricated pentacene thin film transistors on Si/SiO2 and varied the conduction channel thickness from effectively bulk (~40nm) to 2 continuous conducting layers to examine the effect of substrate on noise generation. The electronic spectral noise was measured and the generator of the noise was determined to be due to the random spatial dependence of grain boundaries, independent of proximity to the gate oxide. This result led me to investigate the mechanisms of pentacene grain formation, including the role of small quantities of impurities, on silicon dioxide substrates. Through a series of nucleation, growth and morphology studies, I determined that impurities assist in nucleation on SiO2, decreasing the stable nucleus size by a third and increasing the overall number of grains. The pentacene growth and morphology studies prompted further exploration of pentacene crystal growth on SiO2. I developed a method of making atomically clean ultra-thin oxide films, with surface chemistry and growth properties similar to the standard thick oxides. These ultra-thin oxides were measured to be as smooth as cleaned silicon and then used as substrates for scanning tunneling microscopy of pentacene films. The increased spatial resolution of this technique allowed for the first molecular resolution characterization of the standing-up pentacene crystal structure near the gate dielectric, with molecules oriented perpendicular to the SiO2 surface. Further studies probed how growth of C60 films on SiO2 and pentacene surfaces affected C60 morphology and electronic structure to better understand solar cell heterojunctions.
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    Microbial Ecology and Horticultural Sustainability of Organically and Conventionally Managed Apples
    (2008) Ottesen, Andrea; Walsh, Christopher S; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Objectives: Organically and conventionally managed apple trees (Malus domestica Borkh) were evaluated for three growing seasons (2005-2007) to examine the impact of organic and conventional pesticide applications on the microbial ecology of phyllosphere and soil microflora. An important objective was to establish if organic or conventional selection pressures contribute to an increased presence of enteric pathogens in phyllosphere microflora. The horticultural and economic sustainability of the organic crop was also compared to the conventional crop with regard to fruit yield and input costs. Methods: Microbial populations from phyllosphere and soil environments of apple trees were evaluated using clone libraries of 16S rRNA gene fragments. Clones were sequenced and software was used to assess diversity indices, identify shared similarities and compute statistical differences between communities. These measurements were subsequently used to examine treatment effects on the microbial libraries. Phyllosphere Results: Eight bacterial phyla and 14 classes were found in this environment. A statistically significant difference between organically and conventionally managed phyllosphere bacterial microbial communities was observed at four of six sampling time points. Unique phylotypes were found associated with each management treatment but no increased human health risk could be associated with either treatment with regard to enteric pathogens. Soil Results: Seventeen bacterial phyla spanning twenty-two classes, and two archaeal phyla spanning eight classes, were seen in the 16S rRNA gene libraries of organic and conventional soil samples. The organic and conventional soil libraries were statistically different from each other although the sampling depth was not sufficient to make definitive inference about this environment. Horticultural Results: Fruit yields from organically managed apple trees were from one half to one third of the yields from conventionally managed trees. Based on input costs, organic fruit was about twice as expensive to produce. Asian pears (Prunus serotina) were also included in this horticultural analysis and showed greater field tolerance as an organic specialty niche crop than apples.
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    Novel Organic Polymeric and Molecular Thin-Film Devices for Photonic Applications
    (2006-12-08) Kim, Younggu; Lee, Chi H.; Herman, Warren N.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The primary objective of this thesis is to explore the functionalities of new classes of novel organic materials and investigate their technological feasibilities for becoming novel photonic components. First, we discuss the unique polarization properties of optical chiral waveguides. Through a detailed experimental polarization analysis on planar waveguides, we show that eigenmodes in planar chiral-core waveguides are indeed elliptically polarized and demonstrate waveguides having modes with polarization eccentricity of 0.25, which agrees very well with recent theory. This is, to the best of our knowledge, the first experimental demonstration of the mode ellipticities of the chiral-core optical waveguides. In addition, we also examine organic magneto-optic materials. Verdet constants are measured using balanced homodyne detection, and we demonstrate organic materials with Verdet constants of 10.4 and 4.2 rad/T · m at 1300 nm and 1550 nm, respectively. Second, we present low-loss waveguides and microring resonators fabricated from perfluorocyclobutyl copolymer. Design, fabrication and characterization of these devices are addressed. We demonstrate straight waveguides with propagation losses of 0.3 dB/cm and 1.1 dB/cm for a buried channel and pedestal structures, respectively, and a microring resonator with a maximum extinction ratio of 4.87 dB, quality factor Q = 8554, and finesse F = 55. In addition, from a microring-loaded Mach-Zehnder interferometer, we demonstrate a modulation response width of 30 ps and a maximum modulation depth of 3.8 dB from an optical pump with a pulse duration of 100 fs and a pulse energy of 500 pJ when the signal wavelength is initially tuned close to one of the ring resonances. Finally, we investigate a highly efficient organic bulk heterojunction photodetector fabricated from a blend of P3HT and C60. The effect of multilayer thin film interference on the external quantum efficiency is discussed based on numerical modeling. We experimentally demonstrate an external quantum efficiency ηEQE=87±2% under an applied bias voltage V = −10 V, leading to an internal quantum efficiency ηIQE≈97%. These results show that the charge collection efficiency across the intervening energy barriers can indeed reach near 100% under a strong electric field.